Polishing processes, such as chemical-mechanical polishing (“CMP”), are used in the manufacturing of microelectronic devices to form flat surfaces on semiconductor wafers, field emission displays, and many other microelectronic substrates. For example, the manufacture of semiconductor devices generally involves the formation of various process layers, selective removal or patterning of portions of those layers, and deposition of yet additional process layers above the surface of a semiconducting substrate to form a semiconductor wafer. The process layers can include, by way of example, insulation layers, gate oxide layers, conductive layers, and layers of metal or glass, etc. It is generally desirable in certain steps of the wafer process that the uppermost surface of the process layers be planar, i.e., flat, for the deposition of subsequent layers. Polishing is used to planarize process layers wherein a deposited material, such as a conductive or insulating material, is polished to planarize the wafer for subsequent process steps.
In a typical polishing process (e.g., a CMP process), a substrate is mounted upside down on a carrier in a polishing tool. A force pushes the carrier and the wafer downward toward a polishing pad. The carrier and the wafer are rotated above the rotating polishing pad on the polishing tool's polishing table. A polishing composition, also known as a polishing slurry, generally is introduced between the rotating wafer and the rotating polishing pad during the polishing process. Polishing slurries are widely available, and, as a consumable component of the polishing process, typically must be replenished. To transport and store the slurries, they are provided in a variety of containers ranging in shape and capacity.
For physically removing portions of the process layers, polishing slurries often contain an abrasive material in an aqueous solution. Typical abrasive materials comprise metal oxides (e.g., silicon dioxide, cerium oxide, aluminum oxide, zirconium oxide, tin oxide, coformed particles thereof, and combinations thereof), diamond, boron nitrides, silicon carbide, polymeric materials, and combinations thereof. U.S. Pat. No. 5,527,423, for example, describes a method for chemically-mechanically polishing a metal layer by contacting the surface with a polishing slurry comprising high purity fine metal oxide particles in an aqueous medium. The type of abrasive material selected will depend on the type of substrate being polished. The size of the abrasive particles suspended in the slurry generally ranges from about 1 nanometer (e.g. 1×10−9 meters) to several microns (e.g. 1×10−6 meters), or more. While particles on the order of 20 nanometers to a micron typically function well as abrasives, it has been found that larger abrasive particles (e.g., agglomerates of abrasive particles) may scratch or cause other defects on the polished surfaces of the substrate. Similarly, abrasive particles having a density or hardness significantly greater than the average density or hardness of the abrasive particles can also produce defects during polishing of substrate surfaces. The presence of such defects can ruin the substrate or substantially impair the performance of the finished semiconductor device. To reduce surface defectivity, it is desirable to eliminate undesirably large or dense abrasive particles from the polishing slurry. Therefore, a major focus on the part of slurry manufactures has been to reduce the number of large and/or dense abrasive particles, typically by better controlling the particle production process, dispersion process, or filtering process. Despite these efforts, larger and/or denser particles continue to be present in slurries and surface defectivity remains a problem.